A Mixed-Conduction Model for the Oxidation of Stainless Steel in a High-Temperature Electrolyte

Abstract

The oxide films formed on AISI 316L͑NG͒ in the temperature range 150-300°C have been characterized by impedance spectroscopy and ex situ analysis using Auger electron spectroscopy. Relatively thick films containing a high concentration of mobile defects form on stainless steel in a high-temperature borate electrolyte, but their impedance response is most probably controlled by the properties of a thin barrier sublayer. The ability of the mixed conduction model for passive films to reproduce the experimental impedance data in both alloy/oxide/electrolyte and alloy/oxide/inert metal configurations has been tested. A procedure for the calculation of the kinetic constants of the interfacial reactions of point defect generation/consumption, as well as those characterizing the transport rates of ionic/electronic defects in the oxide, has been developed. The effect of temperature on the kinetic and transport parameters has been assessed, and the relevance of these parameters for the corrosion behavior of stainless steel in a high-temperature electrolyte is discussed. The results show that the nature of the barrier layer does not change drastically with temperature, although the growth mechanism of the oxide film is different at 150-300°C than at room temperature.The materials used in contact with the coolant in nuclear power plants rely almost exclusively on a passivating oxide film to ensure durability and structural integrity. A slow and well-controlled growth of the passive film is necessary in order to limit the impact of the coolant on these materials and to minimize the concentration of impurities that may reach the nuclear fuel surfaces and thus become radioactive.The development of knowledge of the oxide film composition and behavior, as well as the knowledge of the local coolant chemistries, has converged considerably during the last few years. 1,2 It is hence known that the oxide films formed on stainless steels and nickel-based alloys in high-temperature water have basically the same main structure and that the important compounds ensuring the protective character of the oxide film ͑nickel ferrite and nickel and iron chromites͒ are essentially the same for all the nickel-based materials and stainless steels. [3][4][5][6][7][8][9][10][11][12][13][14][15] In addition to the increased understanding of growth and restructuring of oxide films, recent development of radiolysis codes allows the modeling of the chemistry resulting from the radiolysis in virtually any location where the coolant is in contact with any material in light water reactors. The modeling provides the concentration of all radiolysis products, including radicals, and also calculates the local electrochemical corrosion potential in each location, hence providing the basis for determining or establishing thermodynamic, kinetic, and electrokinetic phenomena. 16 We hence have the qualitative understanding of most of the important processes during film growth and restructuring, with their background and implications. 17,18 What we still lack today i...

Kinetic parameters of the oxidation of zirconium alloys in simulated WWER water – Effect of KOH content

“…According to the PDM and the MCM the transport of oxygen vacancies in the oxide is governed by both diffusion and migration [22][23][24][25]: pores of the outer layer and x = L at the alloy/barrier layer interface, L being the barrier layer thickness, gives the concentration profile of oxygen vacancies in the barrier layer:…”

“…On the other hand, some of us have adapted the mixed-conduction model (MCM) for oxide films [24,25] to quantitatively describe the in situ impedance data for several Zircaloy 2-type alloys in simulated boiling water reactor (BWR) coolant at 300°C [20]. As a consequence, several important kinetic and transport parameters of the growing oxide have been estimated.…”

Semiconducting behavior of passive film formed on stainless steel in borate buffer solution containing sulfide

“…The dissolution process of chromium oxides in the passive film was discussed and it was believed that this process led to the formation of duplex structure of passive film, namely the rich-chromium inner layer and poor-chromium outer layer. Bojinov [8,9] put forward a new model of passive film on Fe-Cr alloys through the measurement of contact resistance, photoelectrochemical method and electrochemical impedance spectroscopy (EIS). He believed that this kind of passive film was composed of one main film and two highly defect interface regions beside it.…”

Mixed-Conduction Model for Stainless Steel in a High-Temperature Electrolyte: Estimation of Kinetic Parameters of Inner Layer Constituents

“…The ability of the mixed-conduction model ͑MCM͒ for passive films [5][6][7][8] to reproduce the experimental impedance data in both configurations has been verified. The MCM is at the conceptual level quite similar to the point defect model ͑PDM͒ developed by Zhang and Macdonald 9,10 and Macdonald 11,12 over the last few decades.…”

“…The fact that the impedance of this film does not change significantly with temperature suggested that its electrical and electrochemical properties remain qualitatively unaltered in the entire studied temperature interval. In general, the impedance and surface analytical results reported earlier by some of the authors 5 indicate that regardless of the fact that comparatively thick oxide layers are formed especially at temperatures exceeding 200°C, the kinetics of the overall film growth and corrosion process is governed by a thin barrier layer. In that respect, this quantitative treatment based on a conceptual model consisting of a very thin barrier layer adjacent to the alloy substrate and a much thicker layer growing by dissolution-redeposition has been found to be in full quantitative agreement with the electrochemical impedance data measured after short oxidation times.…”